In a fuel cell the chemical energy of a fuel (typically hydrogen) is transformed directly into electrical energy. A fuel cell consists of an electrolyte that can conduct ions but not electrons, and two electrodes where the chemical reactions occur. As long as fuel is supplied, the cell can operature continuously without need for recharging.
The two electrodes function as catalysts, i.e. they enable the chemical reactions without being consumed themselves. The specific reactions occurring depend on the type of fuel cell. In a PEMFC hydrogen (H2) is transformed into positively charged hydrogen ions (H+) at the anode. The hydrogen ions pass through the electrolyte, and at the cathode they react with oxygen to form water. In an SOFC the cathode facilitates the transformation of oxygen (O2) into negatively charged oxygen ions (O2−) which pass through the electrolyte and react with hydrogen at the anode, forming water.
The electrons left over from the reaction at the anode are unable to pass through the electrolyte. In this way an electrical potential difference arises between the two sides of the cell. The potential difference may be used to drive an electrical current in an external circuit.
Advantages of fuel cells
High efficiency. The direct transformation of chemically bound energy to electrical energy allows fuel cells to have a higher efficiency than ordinary internal combustion engines or gas turbines. This is due to the fact that these rely on a transformation to thermal energy as an intermediate step before the energy is transformed into mechanical work and further into electricity. This gives rise to thermodynamic limitations which fuel cells escape.
Low CO2 emissions. If hydrogen is used as fuel in a fuel cell, the only waste product will be water. But hydrogen does not exist in free form in nature and must be manufactured either from hydrocarbons or by water electrolysis. Both processes involve CO2 emissions (unless the electricity for the electrolysis comes from renewable energy or nuclear energy). And if natural gas is used directly as a fuel, CO2 will also be produced in the cell. However, the CO2 emissions per kWh of electrical energy produced is lower than in ordinary power plants.
No NOx . The temperature in a fuel cell is low compared to flame temperatures. This means that polluting nitrogen oxides (NOx) are not produced.
Low noise. There are no moving parts in the cells themselves, and few in the rest of the system. This means that fuel cell systems make less noise than internal combustion engines or gas turbines.
Modular structure. Both large and small systems have approximately the same price per kilowatt installed effect, and have the same high efficiency. In contrast to internal combustion engines the efficiency remains high, even when not running at full load.
Materials Properties
The electrolyte must be a good ionic conductor but must not conduct electrons (else the cell would short circuit). The two electrodes must be good electronic conductors and in addition be porous to allow gas to enter them and react. It is important that the porous structure is such that the largest possible amount of contact exists between the three phases: ion conducting electrolyte, electron conducting electrode and the reactants of the gas phase. The reactions of the fuel cell occur at these so-called three phase boundaries: Both the cathode and the anode processes involve ions, electrons and gas molecules.
The Microscopics of the Cell Components
To make the following concrete, we will concentrate on the SOFC type.
The electrolyte in an SOFC most often consists of yttria stabilized zirconia, YSZ. Zirconia (zirconium oxide, ZrO2) at high temperatures exists in a cubic crystal structure, as shown below. At lower temperatures the structure is monoclinic or tetragonal.
The crystal structure of cubic zirconia. Oxygen is blue, zirconium is green
By doping zirconia with, e.g., 8 molar pct. of yttria (yttrium oxide, Y2O3), the cubic phase may be stabilized at lower temperatures; the resulting compound is called YSZ. When trivalent Y is substituted for tetravalent Zr, holes (unfilled positions) in the oxygen sub-lattice are introduced at the same time. This makes it possible for oxygen ions to move through the solid by hopping from hole to hole in the lattice. YSZ is therefore a good oxygen ion conductor. In addition the electronic conductivity is very low, making YSZ well suited as an electrolyte.
The cathode may consist of the electronically conducting compound LSM (lanthanum-strontium-manganese oxide, LaxSr1-xMnO3) which can split oxygen molecules by a catalytic process. The net reaction is:
O2 + 4e– → 2O2–.
To increase the length of the three phase boundaries, the cathode can be designed as a porous network of LSM and YSZ.
Schematics of the cathode in an SOFC. A possible reaction path for the cathode reaction is indicated
The created oxygen ions are transported through the electrolyte and react with the fuel at the anode.
The anode is often made of a porous structure of the ceramic YSZ and metallic nickel (a so-called cermet). It is made by mixing YSZ and nickel oxide. The first time the cell is exposed to fuel (hydrogen), the nickel oxide will be reduced to free (metallic) nickel, and at the same time create a porous network because of the decrease of its volume. This anode structure gives a large three phase boundary. The net reaction at the anode is:
2H2 + 2O2– → 2H2O + 4e–.
The two half reactions combine to give the following net reaction for the whole cell:
2H2 + O2 → 2H2O + electricity + heat.
Schematics of the anode in an SOFC. A possible reaction path for the anode reaction is indicated
Electromotive force
The electrochemical reactions in the cell occur spontaneously, the corresponding change in Gibbs free energy being negative. This means that electrons are effectively being moved from the cathode side to the anode side, against the potential difference which builds up. When the fuel cell is not connected to an external circuit, the size of the potential difference (this is called the electromotive force, E) is determined by balancing the change in Gibbs free energy with the electrostatic energy gained by the electrons moving from cathode to anode:
ΔG + nFE = 0,
where ΔG is the change in Gibbs free energy per mol, n (= 4) is the number of electrons involved in the net reaction, and F (= 96485 coulomb/mol) is the Faraday constant (its value being the charge of 1 mol of electrons). When numbers for the specific reactions are put in, the result for all types of fuel cells is an electromotive force around 1 volt.
The potential distribution inside an (ideal) SOFC under open circuit conditions.
The electromotive force is E = V4 – V1.
It may be mentioned that in a real system the voltage under open circuit conditions is smaller than the electromotive force. This is due to imperfections, such as leaks allowing oxygen and hydrogen to react directly. The actual voltage, called the open circuit voltage (OCV), should therefore strictly speaking be distinguished from E. However, in the discussion below we assume them to be equal.
A Fuel Cell under Load
When current is drawn on an SOFC, electrons are removed from the anode into the external circuit. This shifts the chemical equilibrium at the electrodes, causing oxygen ions to be consumed at the anode side. The anode reaction removes the negatively charged oxygen ions from the thin boundary layer in the electrolyte adjoining the anode. The created electrons (also negatively charged) cannot enter the electrolyte, and this means that a positive charge is built up in the electrolyte boundary layer. Correspondingly, a negative charge builds up in the electrolyte boundary layer adjoining the cathode where the oxygen ions are produced. The two charged layers give rise to a potential difference across the electrolyte, driving the oxygen ions through it.
The voltage under load is smaller than the electromotive force: There is a voltage drop associated with the resistance of the electrolyte and the electrodes, and there are losses connected to the chemical reactions both at the anode and at the cathode. The total losses are summarized in an area specific resistance, ASR, given in ohm·cm2 (dividing by the cell area gives you the total cell resistance). This is an extremely important parameter for a fuel cell. Actually, the price per kilowatt electrical effect is directly proportional to ASR (other things being equal). This is due to the fact that the connection between effect density P (watt/cm2), electromotive force and current density i (ampere/cm2) is given by
P = E·i – ASR·i2,
while the cell voltage U is
U = E - ASR·i.
The potential distribution in an SOFC under load. The oxygen ions are driven by the potential difference V2 – V3. The cell voltage U = V4 – V1 is smaller than the electromotive force E because of internal losses.
Many of the processes contributing to the ASR are activated processes, e.g. the hopping conduction of the oxygen ions in the electrolyte. This means that the ASR increases steeply with decreasing temperature, making it a major challenge to lower the operation temperature.
Stacks
As we have seen, a single fuel cell only gives a voltage of approx. 1 volt. To have technologically useful voltages it is therefore necessary to connect many cells in series in a so-called stack. Geometrically the cells may have a variety of forms, e.g. flat plates or hollow tubes. The electrically optimal way is to use flat cells that are stacked. This gives the lowest internal electrical losses by minimizing the current path in the cell.
Schematics of a fuel cell stack. The spacer layers are needed to have a free volume for the flow of fuel and air; the spacers can also be integrated in the interconnect. The sealing is only shown for the lower cell. Other flow configurations, where fuel and air flow in parallel or opposite directions, are also possible
The flat plate cells can have different designs: The most usual is quadratic or rectangular cells as shown on the figure, but round cells are also used, sometimes with a central hole where the fuel flows. Other geometries, such as tubular cells, are used to a lesser extent. Such cells may have manufacturing advantages and can make the sealing separating fuel and air easier to do. At Risø we are using flat plate cells. You may read more about our stack development here.
Efficiency
The efficiency of a machine (or a power plant) is the ratio between the useful work the machine can perform and the amount of energy it must be supplied with to do it. Often the efficiency is calculated with respect to the higher heating value (HHV) of the fuel used. The higher heating value is the heat of combustion per mol when both the reactants and the reaction products are at 25 °C (298 K) and a pressure of 1 atm.
For a machine having thermal energy as an intermediate step, the highest achievable efficiency is equal to the efficiency of the Carnot cycle:
η = (T–T0)/T, where T is the operating temperature and T0 is the temperature of the exhaust gasses (in this case 25 °C). For a fuel cell the efficiency depends on the operating temperature and the fuel as shown on the graph below.
Of course, in practice the theoretical efficiency is not realised because of the internal losses in the cells. The actual efficiency is a product of three factors: the theoretical efficiency, the voltage efficiency (defined as the ratio of the actual cell voltage to the electromotoric force E) and the fuel utilization α (defined as the ratio of the amount of fuel that reacts electrochemically to the amount of fuel entering the cell):
η = ηt·ηv·α.
In addition there are losses in the rest of the system, e.g. when hydrocarbons are reformed to hydrogen. For the low temperature fuel cells AFC, PEMFC og PAFC the actual efficiency can be around 40%, while for the high temperature fuel cells MCFC og SOFC it can reach approx. 60%. If the waste heat from the latter are used to run a gas turbine, the combined electrical efficiency may be as high as 80-85% |